Re: [PATCHv4 3/4] x86/tdx: Dynamically disable SEPT violations from causing #VEs

[Nikolay Borisov <nik.borisov@suse.com>]

On 12.05.24 г. 15:21 ч., Kirill A. Shutemov wrote:
> Memory access #VE's are hard for Linux to handle in contexts like the
> entry code or NMIs.  But other OSes need them for functionality.
> There's a static (pre-guest-boot) way for a VMM to choose one or the
> other.  But VMMs don't always know which OS they are booting, so they
> choose to deliver those #VE's so the "other" OSes will work.  That,
> unfortunately has left us in the lurch and exposed to these
> hard-to-handle #VEs.
> 
> The TDX module has introduced a new feature.  Even if the static
> configuration is "send nasty #VE's", the kernel can dynamically request
> that they be disabled.
> 
> Check if the feature is available and disable SEPT #VE if possible.
> 
> If the TD allowed to disable/enable SEPT #VEs, the ATTR_SEPT_VE_DISABLE
> attribute is no longer reliable. It reflects the initial state of the
> control for the TD, but it will not be updated if someone (e.g. bootloader)
> changes it before the kernel starts. Kernel must check TDCS_TD_CTLS bit to
> determine if SEPT #VEs are enabled or disabled.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Fixes: 373e715e31bf ("x86/tdx: Panic on bad configs that #VE on "private" memory access")
> Cc: stable@vger.kernel.org

Reviewed-by: Nikolay Borisov <nik.borisov@suse.com> though one nit below.

> ---
>   arch/x86/coco/tdx/tdx.c           | 88 +++++++++++++++++++++++++------
>   arch/x86/include/asm/shared/tdx.h | 11 +++-
>   2 files changed, 83 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 1ff571cb9177..ba37f4306f4e 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -77,6 +77,20 @@ static inline void tdcall(u64 fn, struct tdx_module_args *args)
>   		panic("TDCALL %lld failed (Buggy TDX module!)\n", fn);
>   }
>   
> +/* Read TD-scoped metadata */
> +static inline u64 tdg_vm_rd(u64 field, u64 *value)
> +{
> +	struct tdx_module_args args = {
> +		.rdx = field,
> +	};
> +	u64 ret;
> +
> +	ret = __tdcall_ret(TDG_VM_RD, &args);
> +	*value = args.r8;
> +
> +	return ret;
> +}

nit: Perhaps this function can be put in the first patch and the 
description there be made more generic, something along the lines of 
"introduce functions for tdg_rd/tdg_wr" ?

> +
>   /* Write TD-scoped metadata */
>   static inline u64 tdg_vm_wr(u64 field, u64 value, u64 mask)
>   {
> @@ -179,6 +193,62 @@ static void __noreturn tdx_panic(const char *msg)
>   		__tdx_hypercall(&args);
>   }
>   
> +/*
> + * The kernel cannot handle #VEs when accessing normal kernel memory. Ensure
> + * that no #VE will be delivered for accesses to TD-private memory.
> + *
> + * TDX 1.0 does not allow the guest to disable SEPT #VE on its own. The VMM
> + * controls if the guest will receive such #VE with TD attribute
> + * ATTR_SEPT_VE_DISABLE.
> + *
> + * Newer TDX module allows the guest to control if it wants to receive SEPT
> + * violation #VEs.
> + *
> + * Check if the feature is available and disable SEPT #VE if possible.
> + *
> + * If the TD allowed to disable/enable SEPT #VEs, the ATTR_SEPT_VE_DISABLE
> + * attribute is no longer reliable. It reflects the initial state of the
> + * control for the TD, but it will not be updated if someone (e.g. bootloader)
> + * changes it before the kernel starts. Kernel must check TDCS_TD_CTLS bit to
> + * determine if SEPT #VEs are enabled or disabled.
> + */
> +static void disable_sept_ve(u64 td_attr)
> +{
> +	const char *msg = "TD misconfiguration: SEPT #VE has to be disabled";
> +	bool debug = td_attr & ATTR_DEBUG;
> +	u64 config, controls;
> +
> +	/* Is this TD allowed to disable SEPT #VE */
> +	tdg_vm_rd(TDCS_CONFIG_FLAGS, &config);
> +	if (!(config & TDCS_CONFIG_FLEXIBLE_PENDING_VE)) {
> +		/* No SEPT #VE controls for the guest: check the attribute */
> +		if (td_attr & ATTR_SEPT_VE_DISABLE)
> +			return;
> +
> +		/* Relax SEPT_VE_DISABLE check for debug TD for backtraces */
> +		if (debug)
> +			pr_warn("%s\n", msg);
> +		else
> +			tdx_panic(msg);
> +		return;
> +	}
> +
> +	/* Check if SEPT #VE has been disabled before us */
> +	tdg_vm_rd(TDCS_TD_CTLS, &controls);
> +	if (controls & TD_CTLS_PENDING_VE_DISABLE)
> +		return;
> +
> +	/* Keep #VEs enabled for splats in debugging environments */
> +	if (debug)
> +		return;
> +
> +	/* Disable SEPT #VEs */
> +	tdg_vm_wr(TDCS_TD_CTLS, TD_CTLS_PENDING_VE_DISABLE,
> +		  TD_CTLS_PENDING_VE_DISABLE);
> +
> +	return;
> +}
> +
>   static void tdx_setup(u64 *cc_mask)
>   {
>   	struct tdx_module_args args = {};
> @@ -204,24 +274,12 @@ static void tdx_setup(u64 *cc_mask)
>   	gpa_width = args.rcx & GENMASK(5, 0);
>   	*cc_mask = BIT_ULL(gpa_width - 1);
>   
> +	td_attr = args.rdx;
> +
>   	/* Kernel does not use NOTIFY_ENABLES and does not need random #VEs */
>   	tdg_vm_wr(TDCS_NOTIFY_ENABLES, 0, -1ULL);
>   
> -	/*
> -	 * The kernel can not handle #VE's when accessing normal kernel
> -	 * memory.  Ensure that no #VE will be delivered for accesses to
> -	 * TD-private memory.  Only VMM-shared memory (MMIO) will #VE.
> -	 */
> -	td_attr = args.rdx;
> -	if (!(td_attr & ATTR_SEPT_VE_DISABLE)) {
> -		const char *msg = "TD misconfiguration: SEPT_VE_DISABLE attribute must be set.";
> -
> -		/* Relax SEPT_VE_DISABLE check for debug TD. */
> -		if (td_attr & ATTR_DEBUG)
> -			pr_warn("%s\n", msg);
> -		else
> -			tdx_panic(msg);
> -	}
> +	disable_sept_ve(td_attr);
>   }
>   
>   /*
> diff --git a/arch/x86/include/asm/shared/tdx.h b/arch/x86/include/asm/shared/tdx.h
> index fdfd41511b02..fecb2a6e864b 100644
> --- a/arch/x86/include/asm/shared/tdx.h
> +++ b/arch/x86/include/asm/shared/tdx.h
> @@ -16,11 +16,20 @@
>   #define TDG_VP_VEINFO_GET		3
>   #define TDG_MR_REPORT			4
>   #define TDG_MEM_PAGE_ACCEPT		6
> +#define TDG_VM_RD			7
>   #define TDG_VM_WR			8
>   
> -/* TDCS fields. To be used by TDG.VM.WR and TDG.VM.RD module calls */
> +/* TDX TD-Scope Metadata. To be used by TDG.VM.WR and TDG.VM.RD */
> +#define TDCS_CONFIG_FLAGS		0x1110000300000016
> +#define TDCS_TD_CTLS			0x1110000300000017
>   #define TDCS_NOTIFY_ENABLES		0x9100000000000010
>   
> +/* TDCS_CONFIG_FLAGS bits */
> +#define TDCS_CONFIG_FLEXIBLE_PENDING_VE	BIT_ULL(1)
> +
> +/* TDCS_TD_CTLS bits */
> +#define TD_CTLS_PENDING_VE_DISABLE	BIT_ULL(0)
> +
>   /* TDX hypercall Leaf IDs */
>   #define TDVMCALL_MAP_GPA		0x10001
>   #define TDVMCALL_GET_QUOTE		0x10002